Sheathed Element Glow Plug Having an Integrated Pressure Measuring Element

ABSTRACT

In combustion chamber signal-based engine control in particular, a reliable and at the same time cost-effective option for detecting a combustion chamber pressure in compression-ignition internal combustion engines is required. A sheathed element glow plug is proposed, which has a glow plug module and a pressure measuring module connected to the glow plug module. The glow plug module has a heating element and a glow plug housing. The pressure measuring module has at least one force measuring element, which is integrated into the pressure measuring module. The at least one force measuring element is designed to generate an electrical signal as a function of a force. In the mounted state of the sheathed element glow plug, the at least one force measuring element is connected to the heating element in such a way that a force is transmittable to the at least one force measuring element via the heating element.

FIELD OF THE INVENTION

The present invention relates to a sheathed element glow plug having an integrated pressure measuring element and a method for manufacturing a sheathed element glow plug of this type. In particular, the present invention relates to a sheathed element glow plug which, in addition to a heating element and a glow plug housing, has a pressure measuring module having at least one force measuring element.

BACKGROUND INFORMATION

As a result of increasingly strict legal exhaust gas regulations, in particular for diesel engines, the requirements for reduced emissions of harmful substances by compression-ignition internal combustion engines are becoming stricter. Modern engine management systems should ensure low fuel consumption, while having a long service life.

Combustion in the combustion chamber of a diesel engine may be optimized in particular by using regulated injection of fuel. This regulated injection may be controlled in particular by electronic engine control units, which are already routinely used in today's motor vehicles. Successful operation of a combustion signal-based control system (CSC) depends, however, on the availability of pressure sensors that are industrially manufacturable and meet high demands regarding price, reliability, accuracy, and compactness.

At this time, measuring devices having so-called “stand-alone sensors” are widely used. However, for using such devices, a separate bore hole must be provided in the cylinder head wall, which means additional assembly effort. In addition, stand-alone sensors require additional bore holes in the cylinder head of the internal combustion engine, which, in particular in the case of four-valve engines as designed today, raises considerable problems due to the very scarce available space. Furthermore, in general, systems of this type are relatively expensive and the service life of systems of this type is significantly shorter than that of a typical vehicle, mostly due to the high operating temperatures.

Therefore, there have been approaches in the related art to integrate combustion chamber pressure sensors into existing components of the cylinder head. Examples of such integration are spark plugs having an integrated piezoelectric force measuring element which are known, for example, from DE 694 05 788 T2. This document describes a spark plug having a built-in pressure sensor, the pressure sensor having at least one pressure introduction channel which connects the combustion chamber of a corresponding cylinder of the internal combustion engine to the pressure sensor.

Such a sensor integration into existing components of the cylinder head results in a substantial price advantage and also makes industrial application economically feasible. In systems of this type there is no need to directly access the combustion chamber. On the other hand, the signal quality of such combustion chamber pressure sensors is strongly dependent on the variation of forces in the entire mechanical complex and usually does not meet the requirements that have been set.

German Patent Application No. DE 196 80 912 C2 discloses a device and a method for detecting the cylinder pressure in a diesel engine. The device has a pressure sensor, a heating section of a glow plug, which is installed in the interior of a cylinder of the diesel engine and is exposable to the cylinder pressure, and a fastening element for fastening the heating section in a body of the glow plug. The pressure sensor is situated between the heating section and the fastening element of the glow plug. The cylinder pressure is transmitted to the pressure sensor via the heating section.

Furthermore, the device described in German Patent Application No. DE 196 80 912 C2 has the disadvantage in particular that the pressure sensor is directly connected to the heating section and is therefore exposed to high thermal stresses and temperature fluctuations. Furthermore, assembly of the device described in German Patent Application No. DE 196 80 912 C2 is complicated because the heating section must first be introduced into the body of the glow plug, whereafter the pressure sensor must be subsequently centrally positioned on the heating section to ensure faultless functionality of the device. Subsequently, the pressure sensor must be fastened to the side of the device facing away from the cylinder using the fastening device, which in turn carries the risk of a displacement of the pressure sensor. Testing the functionality of the pressure sensor is not possible until the glow plug is completely assembled.

German Patent Application No. DE 102 18 544 A1 describes a glow plug having a built-in combustion chamber pressure sensor. The glow plug has a housing having a combustion chamber pressure sensor, which is situated concentrically around a core and is pre-stressed by the core. The combustion chamber pressure in an engine is transmitted via the core as a force, whereby the force exerted on the pressure sensor via the core is diminished. The combustion chamber pressure is thus converted into a reduction in a pre-load on the pressure sensor and may thus be detected with the aid of an electrical signal of the pressure sensor.

The system described in German Patent Application No. DE 102 18 544 has, however, the disadvantage of a large number of parts. Furthermore, the connection between the core and the pressure sensor situated concentrically around this core may present difficulties in the assembly because stresses may occur between the core and the pressure sensor. Component tolerances may manifest themselves as critical in this case. Furthermore, the design described in German Patent Application No. DE 102 18 544 A1 cannot be tested until assembly is completed, so any errors cannot be recognized until after full assembly.

SUMMARY

Therefore, a sheathed element glow plug having an integrated pressure measuring element and a method for its manufacture is proposed which avoids the disadvantages of the sheathed element glow plug having an integrated pressure measuring element known from the related art.

In accordance with an example embodiment of the present invention, the sheathed element glow plug is assembled from two separate modules. The sheathed element glow plug has a heating element and a glow plug housing, which are components of a first module of the sheathed element glow plug. Furthermore, the sheathed element glow plug has a pressure measuring module adjacent to the glow plug housing on a side facing away from the heating element. At least one force measuring element, capable of generating an electrical signal as a function of a force, is integrated into the pressure measuring module. The pressure measuring module is connected to the first module in such a way that the at least one force measuring element is directly or indirectly connected to the heating element, in such a way that a force is transmittable to the at least one force measuring element via the heating element.

A plurality of conventional components may be used as a force measuring element. Piezoelectric, piezoresistive, capacitive, inductive force measuring elements, and/or measuring elements having strain gages may be advantageously used in particular. However, other mechanical-to-electrical energy conversion elements may also be used.

An example sheathed element glow plug according to the present invention has numerous advantages compared to conventional devices. In particular, the use of a separate pressure measuring module and the modular design have the effect that the external dimensions of the at least one force measuring element are no longer limited by the internal space of the glow plug housing. This makes a greater selection of force measuring elements commercially available, which helps reduce production costs and avoid delivery bottlenecks. Occasionally force measuring elements having a higher accuracy, lower temperature dependence, and other advantageous properties may also be used, which, however, were not integratable into conventional sheathed element glow plugs due to the reduced space available for installation.

Furthermore, the modular design of the sheathed element glow plug has the effect that a high degree of customer-independence is ensured in manufacturing and assembly, so that, for example, the same type of sheathed element glow plug may be integrated into different engine types. For example, simple replacement of the pressure measuring module is sufficient for this purpose.

In particular, in the manufacture of the sheathed element glow plug the first module and the pressure measuring module may be manufactured by separate manufacturers or in different manufacturing facilities, for example. In manufacturing the first module, the heating element is connected to the glow plug housing. In manufacturing the pressure measuring module, at least one force measuring element is integrated into the pressure measuring module. Both modules are separately testable in particular because the functionality of either module is independent of the other module.

In this way errors may be detected and eliminated early in the manufacturing process, which considerably reduces rejects, lowers costs, and increases the reliability of the components. Subsequently the pressure measuring module and the first module are connected to each other in such a way that a force is transmittable to the at least one force measuring element via the heating element. This connection may be achieved via an integral joint, for example, welding, and/or a friction-locked joint.

At least one force transmission element may be used for the transmission of force from the heating element to the at least one force measuring element. For example, the at least one force transmission element may have a force transmission ram.

This additional at least one force transmission element offers the advantage in particular that the heating element and the at least one force measuring element may be spatially separated. The pressure measuring module may thus be situated outside the combustion chamber of the internal combustion engine, so that the at least one force measuring element is exposed to the least possible thermal stresses and temperature fluctuations. Since the signal characteristic of many commercially available force measuring elements significantly depends on the operating temperature and/or temperature fluctuations, this design according to the present invention is highly advantageous for the signal quality.

It has also been found advantageous if the at least one force measuring element has at least one actuator surface extending perpendicularly to an axis of the sheathed element glow plug. The at least one force measuring element may have the form of a disk, for example, a circular disk, and may be attached radially to a housing of the pressure measuring module. A force may be exerted, directly or indirectly, in the axial direction, on the at least one actuator surface via the at least one force transmission element. For example, the sheathed element glow plug may have at least one additional diaphragm, which is situated between the at least one force transmission element and the at least one force measuring element. This diaphragm may be an integral component of the at least one force measuring element in particular.

In the design in which at least one force transmission element is used, it has been found advantageous if the sheathed element glow plug also has a vibration damping element. This vibration damping element may fill, completely or partially, in particular at least one cavity between the at least one force transmission element and the glow plug housing. In particular, if elongated force transmission elements are used, for example, a force transmission ram, this has been found advantageous for reducing the vibrations of the at least one force transmission element in order to avoid undesirable vibrations and thus undesirable components in the electrical signal of the at least one force measuring element.

The first module and the pressure measuring module may be separated in particular by an interface, for example, a separating wall. In this case it is advantageous if the at least one separating wall has at least one force transmission opening. For example, this force transmission opening may be a central bore hole through which a force transmission ram is guided in the axial direction.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention is explained in greater detail below with reference to the figures.

FIG. 1 shows an exemplary embodiment of a sheathed element glow plug according to the present invention.

FIG. 2 shows an exemplary embodiment of an example method according to the present invention for manufacturing a sheathed element glow plug.

DETAILED DESCRIPTION OF EXAMPLE EMBODIMENTS

FIG. 1 shows a specific embodiment of a sheathed element glow plug 110 according to the present invention. Sheathed element glow plug 110 has a first module 112 (glow plug module) and a pressure measuring module 114. Both modules are separated along an interface by a separating wall 116, which may be a component of pressure measuring module 114 or also of glow plug module 112. Two separating walls 116 are also possible, one separating wall being a component of glow plug module 112 and one separating wall being a component of pressure measuring module 114. Pressure measuring module 114 and glow plug module 112 may be connected by welding, for example. A threaded connection using an appropriate union nut is also conceivable.

Glow plug module 112 has a housing 118 having an essentially cylindrical interior space 120. At one end of housing 118 facing away from pressure measuring module 114 and facing a combustion chamber of the internal combustion engine, housing 118 has a sealing cone 122 into which a (ceramic in this exemplary embodiment) heating element 124 is inserted. Ceramic heating element 124 in sealing cone 122 is connected to housing 118 in such a way, for example, by gluing, that interior space 120 is generally sealed off against the influences of the combustion chamber. Ceramic heating element 124 may also be soldered into a metallic support tube, which is then welded to the housing or pressed thereinto.

Ceramic heating element 124 has a pressure surface 126 on its front end facing the combustion chamber, this pressure surface being designed in this exemplary embodiment as a surface perpendicular to an axis 128 of sheathed element glow plug 110. The pressure in the combustion chamber of the internal combustion engine causes a force F (reference numeral 130) to be exerted on the ceramic heating element (which may also logically be a metallic heating element). This exerted force causes a linear-elastic spring deflection of the components of sheathed element glow plug 110 situated in the force's path. This spring deflection is normally in the micrometer range. A force directly correlating with the combustion chamber pressure may be transmitted to sheathed element glow plug 110 via this linear-elastic spring deflection.

One possible way of detecting this force 130 is, according to an example embodiment of the present invention, by placing a force measuring element 132 in pressure measuring module 114. Force measuring element 132 has basically the shape of a circular disk situated perpendicularly to axis 128 and has an end face 134 having an integrated, miniaturized diaphragm, which is situated on the side facing heating element 124. The force measuring element, which may be, for example, a piezoresistive force measuring element as described above, has two signal lines 138, which exit axially from a housing 140 at the end of pressure measuring module 114 facing away from glow plug module 112 and are connected to an appropriate signal processor (not illustrated).

If a force is exerted on force measuring element 132, it is converted by force measuring element 132 into appropriate electrical signals, for example, a voltage or a current, which may be picked up with the aid of signal lines 138.

In this exemplary embodiment, the periphery of force measuring element 132 is connected to housing 140, for example, by gluing or by an appropriate thread. Additionally or alternatively, force measuring element 132 may also be supported from the side facing away from heating element 124, for example, by housing 140 having an internal thread, into which an appropriate ring having an external thread may be screwed in.

In contrast to the above-described German Patent Application No. DE 196 80 912 C2, in this embodiment of sheathed element glow plug 110 according to the present invention, an additional fastening element behind force measuring element 132 may thus be omitted. Instead, the periphery of force measuring element 132 is fastened to housing 140 of pressure measuring module 114 separated from glow plug module 112. The example embodiment according to the present invention thus differs substantially also from the system described in European Patent Application No. EP 102 18 544 A1. In particular, as described above, separate manufacturing of the two modules 112, 114 is possible. Different variants of glow plug module 112 and pressure measuring module 114, whose geometries are adjusted to specific customer requirements, may also be manufactured. However, the interface geometry, which enables the two modules 112, 114 to be joined along separating wall 116, should be type-independent, so that a “kit” composed of glow plug modules 112 and pressure measuring module 140 results.

Due to the length of force transmission element 142, i.e., in particular the fact that force transmission element 142 has a length that is substantially greater than its diameter, vibration modes in the mostly undesirable low-frequency range may occur in the excitation spectrum for mechanical vibrations. Such self-resonances make the analysis of the signal of force measuring element 132 difficult or completely impossible.

To eliminate this problem, according to the present invention a vibration damping element 146 is provided in the exemplary embodiment according to FIG. 1. In this exemplary embodiment, this vibration damping element 146, in which a cylindrical force transmission ram 142 is used as a force transmission element, is advantageously designed as a cylindrical sleeve and completely fills an annular gap between force transmission ram 142 and housing 118 of glow plug module 112.

To completely fill the annular gap, vibration damping element 146 may advantageously have a resin, in particular an electrically and/or thermally insulating resin. This resin may be filled into the annular gap between force transmission ram 142 and housing 118 of glow plug module 112 in the liquid state and subsequently hardened there. Instead of a resin, a plurality of other materials may also be used, which may be introduced into the annular gap in the liquid or plastic state to be subsequently hardened there. The material may be hardened completely or partially, so that, for example, an elastic or a plastic vibration damping element 146 results. Furthermore, the material may be hardened in different ways, for example, by UV effect or by heat treatment. Numerous other options are conceivable and known to those skilled in the art.

Furthermore, in the exemplary embodiment of FIG. 1, the steel terminal bolt normally used in sheathed element glow plugs 110 for conducting the glow current to ceramic heating element 124 is replaced by a thin and/or flexible wire glow current lead 148. The effect of wire glow current lead 148 is that the reduced installation space available within housing 118 of glow plug module 112 is optimally utilized. This wire glow current lead 148 may exit from housing 118 of glow plug module 112 via a radial bore hole 150 and be connected to an appropriate power supply.

FIG. 2 shows an exemplary embodiment of a method according to the present invention for manufacturing a sheathed element glow plug 110, for example, sheathed element glow plug 110 illustrated in FIG. 1. The method steps illustrated do not necessarily need to be performed in the order illustrated and additional method steps, not illustrated in FIG. 2, may be performed. Individual method steps may also be performed repeatedly.

In method step 210, a first module 112 is manufactured, which has a heating element 124 and a housing 118. Heating element 124 is connected to glow plug housing 118. In method step 212, this first module 112 is tested, the functionality of heating element 124 being checked, for example, by applying an appropriate current, for example, via wire glow current lead 148.

In method step 214, a pressure measuring module 114, for example, according to the exemplary embodiment of FIG. 1, is assembled, in particular at least one force measuring element 132, for example, [force measuring element] 132 of the type described above, being integrated into pressure measuring module 114. Subsequently in method step 216, pressure measuring module 114 is tested. This test may be performed, for example, by applying a defined force to diaphragm 136 of force measuring module 132 through opening 144 in separating wall 116 via a suitable test element; the electrical signal generated by force measuring element 132 may be detected, for example, on signal lines 138. In this way, malfunctions of pressure measuring module 114 may be recognized in particular.

Subsequently in method step 218, pressure measuring module 114 and glow plug module 112 are connected, for example, by welding or with the aid of a union nut. This connection takes place in such a way that a force is transmittable from heating element 124 to force measuring element 132. For example, for this purpose of connection force transmission ram 142 may be guided through central bore hole 144 into the interior of pressure measuring module 114 and placed onto diaphragm 136.

In this exemplary embodiment, force 130 is transmitted from heating element 124 fully or partially with the aid of a force transmission element 142, which in this exemplary embodiment is designed as a force transmission ram 142. This force transmission ram 142 transmits force 130 axially onto end face 134 of force measuring element 132. For this purpose, force transmission ram 142 is guided through a central bore hole 144. 

1-11. (canceled)
 12. A sheathed element glow plug for a compression-ignition internal combustion engine, comprising: a first module containing a heating element and a glow plug housing; and a pressure measuring module, a side of the pressure measuring module which faces away from the heating element being adjacent to the first module, at least one force measuring element being integrated into the pressure measuring module, the at least one force measuring element being adapted to generate an electrical signal as a function of a force, the at least one force measuring element being connected to the heating element in such a way that a force is transmittable to the at least one force measuring element via the heating element.
 13. The sheathed element glow plug as recited in claim 12, wherein the at least one force measuring element includes at least one of a piezoelectric force measuring element, a piezoresistive force measuring element, a capacitive force measuring element, an inductive force measuring element, and a force measuring element having strain gages.
 14. The sheathed element glow plug as recited in claim 12, wherein the glow plug housing and the pressure measuring module are one of connected by an integral joint, or friction-locked to each other.
 15. The sheathed element glow plug as recited in claim 12, further comprising: at least one additional force transmission element, a force being transmittable from the heating element to the at least one force measuring element with the aid of the at least one force transmission element.
 16. The sheathed element glow plug as recited in claim 12, wherein the at least one force transmission element has a force transmission ram.
 17. The sheathed element glow plug as recited in claim 15, wherein the at least one force measuring element has at least one actuator surface extending perpendicularly to an axis of the sheathed element glow plug, the at least one force measuring element is radially attached to a housing of the pressure measuring module, and a force being exertable in an axial direction directly or indirectly onto the at least one actuator surface using the at least one force transmission element.
 18. The sheathed element glow plug as recited in claim 15, further comprising: at least one diaphragm between the at least one force transmission element and the at least one force measuring element, the at least one diaphragm being preferably integrated into the at least one force measuring element.
 19. The sheathed element glow plug as recited in claim 15, further comprising: at least one vibration damping element installed in the glow plug housing, the at least one vibration damping element filling, one of fully or partially, at least one cavity between the at least one force transmission element and the glow plug housing.
 20. The sheathed element glow plug as recited in claim 19, wherein the at least one vibration damping element has at least one resin.
 21. The sheathed element glow plug as recited in claim 12, further comprising: at least one separating wall between the glow plug housing and the pressure measuring module, the at least one separating wall having at least one force transmission opening.
 22. A method for manufacturing a sheathed element glow plug for a compression-ignition internal combustion engine, comprising: assembling a first module having a heating element and a glow plug housing, the heating element being connected to the glow plug housing; assembling a pressure measuring module having at least one force measuring element, the at least one force measuring element adapted to generate an electrical signal as a function of a force; and connecting the pressure measuring module to the first module so that a force is transmittable to the at least one force measuring element via the heating element. 